This invention relates to an angle modulated wave demodulation system, and more particularly to a system for controlling the lock range of a phase locked loop for demodulating an angle modulated wave reproduced from a multichannel disc.
A "phase locked loop" is a filter and demodulator combination using a voltage controlled oscillator. The center frequency of the phase locked loop is determined by the free-running frequency of the voltage controlled oscillator. The "lock range" or tracking range is the range of frequencies on either side of the center frequency which may be produced by the voltage controlled oscillator while it remains locked on the center frequency.
In general, a record disc of a discrete four-channel system carries direct waves comprised of sum signals derived from each of two channels and angle modulated waves resulting from the angle modulation of a carrier wave of 30 KHz by difference signals derived from each of two channels. These signals are superimposed and recorded on the two walls of the disc sound groove, as disclosed in detail in U.S. Pat. No. 3,686,471 entitled "SYSTEM FOR RECORDING AND/OR REPRODUCING FOUR CHANNEL SIGNALS ON A RECORD DISC".
In some cases, an absence of a signal or an abrupt decrease in the level of a signal may be picked up by a phonograph pick-up stylus. This occurs, for example, when a pick-up stylus rides over a particle of dust in a groove of a record disc. Sometimes the pick-up fails to completely trace on walls of the groove. Other times a part of the groove is damaged and part of the waveform is lacking from the groove. If the waveform on the groove is fine and complicated; the pick-up stylus jumps over it and fails to completely trace the waveform. Also, a reproduced part of the groove may be worn away due to repeated reproduction of the record disc, by a pick-up cartridge having a stylus tip with too large an equivalent mass, i.e., a large moment of inertia. As stated above, the frequency range of the angle modulated difference signal (20 KHz to 40 KHz) in the signal recorded on the groove is higher than the range of the direct wave sum signal. Accordingly, the part of the waveform having a relatively long and gradual groove wall curve relates to the direct wave sum signal. Another part of the waveform is relatively small and fine and relates to the angle modulated wave difference signal. Therefore, the absence of a signal or the drop in the level in the reproduced signal usually occurs in the angle modulated wave difference signal.
In the absence of a signal or the drop in the level, the frequency of the carrier component in the angle modulated wave difference signal abruptly deviates to a substantially very low frequency. Accordingly, noise is generated in an angle demodulator in a later stage responsive to the abrupt deviation of the carrier wave to a low frequency.
The abrupt decrease in the angle modulated wave and the resulting generation of noise is a characteristic of reproduction of the multichannel record disc. It is essentially different from a noise generation which generally occurs in FM communication, etc. In FM communication the carrier is always maintained at a constant level and a noise signal is added thereto with a result that the level as a whole increases rather than decreases. Furthermore, responsive to fading, the change in the level of an angle modulated wave is gradual and not abrupt. Whereas in the multi-channel record disc, it is inevitable that the level of the angle modulated wave decreases abruptly, due to the relationship between the tip of the stylus and the groove, existence of dust, damage of the groove and other causes.
When, the input carrier level is abnormally low, as mentioned above, the center frequency of the carrier oscillation frequency does not coincide in the voltage controlled oscillator and in the phase locked loop (PLL). This lack of coincidence results from a fluctuation in the rotational speed of a turntable of a record player or a variation in the temperature of the circuit. The PLL deviates from the locked state and generates beats of a frequency equal to the difference between the above mentioned oscillation frequency and the frequency of the input angle modulated carrier wave. Therefore normal demodulation is no longer carried out. These fluctuations are such that when there is a deviation in the turntable speed in the order of 3 percent or a fluctuation in the frequency of the voltage controlled oscillator or a temperature variation in the order of 3 percent, a frequency difference is produced in the order of 1.8 KHz with respect to a carrier frequency of 30 KHz of the input angle modulated difference signal.
As mentioned above, the PLL holds its locked state with respect to occurrences such as fluctuations in the turntable speed at the time of abnormal deterioration level. In order to prevent generation of beats as mentioned above, the applicant has previously proposed a novel multichannel disc demodulation system as disclosed in U.S. patent application Ser. No. 294,371, filed Oct. 2, 1972 now abandoned, entitled "ANGLE MODULATED WAVE DEMODULATION SYSTEM".
In a system according to our copending application Ser. No. 294,371, the time constant circuit 44 is provided at a position succeeding the phase comparator, as illustrated in FIG. 4 of that copending application. The time constant circuit 44 has a circuit, as there illustrated in FIG. 6.
In that FIG. 6, an input signal is applied to a left-side terminal and an output signal is derived from a right-side terminal. The output signal voltage V is represented by the following formula: ##EQU1## where, E is the voltage of input signal.
The value of 1/(j2.pi.fC.sub.1) is inverse to the value of frequency f, which is a part of the denominator.
The value of capacitance C.sub.1 is selected to obtain the following relationship: ##EQU2## when the frequency f.sub.1 is an audio frequency and ##EQU3## when the frequency f.sub.2 is a much lower frequency (i.e., approaches a direct current).
Accordingly, for audio frequency components f.sub.1, the formula (1) becomes ##EQU4## and the output signal is attenuated to R.sub.2 / (R.sub.1 + R.sub.2) of the input signal.
On the other hand, for frequency components f.sub.2 much lower than the audio frequencies, the equation (1) becomes ##EQU5## and, the output signal V(f.sub.2) substantially is equivalent to the input signal E(f.sub.2).
The lower frequency components in the output signal of the phase comparator occur when a turn-table of a record player rotates at a speed which deviates from the normal speed. When the central oscillation frequency of the voltage controlled oscillator deviates from the central frequency of the modulated wave, due to changes with respect to a lapse of time, the output signal level of the time constant circuit 44 becomes substantially equal to the level of the input signal. Therefore, the same result is obtained irrespective of the presence of the time constant circuit.
Accordingly, the DC lock range of the phase locked loop (the lock range with respect to the lower frequency components) does not relate to the existence or non-existence of the time constant circuit.
The level of the output signal from the time constant circuit becomes smaller than that of the input signal, with respect to the audio frequency components out of the output signal of the phase comparator. The lock range (AC lock range) of the phase locked loop, with respect to the audio frequency components becomes more narrow than the DC lock range.
While this proposed system of U.S. patent application Ser. No. 294,371 obtains a narrow AC lock range, it is accompanied by a problem in that the DC lock range cannot be widened any further. Accordingly, in order to obtain a large ratio of the DC lock range and the AC lock range (hereinafter referred to simply as "lock range ratio"), the AC lock range must be made small since the DC lock range is constant and cannot be made greater than a certain value. However, this measure is not desirable owing to the following point.
When the level of the angle modulated difference signal is high, and any harmonics component of the direct wave sum signal coming into the band of the former signal can be neglected, it is necessary to carry out demodulation with an enlarge lock range of the PLL. On the other hand, when there is a high level of noise due to causes such as wear of the disc sound groove, or when there is much disturbance due to causes such as the harmonics component of the direct wave sum signal, it is necessary to carry out demodulation with a small PLL lock range. For this reason, it is desirable that the above mentioned lock range ratio be large. That is, when the quality of the reproduced angle modulated difference signal is good the PLL lock range is expanded to carry out demodulation with a wide band and with high fidelity. When the quality is poor, the PLL lock range is made narrow to demodulate the signal of only a specific frequency range.
If the above mentioned lock range ratio has a large value (for example, approximately 10:1), the maximum value of the AC lock range will also become small in the above mentioned proposed system, and high-fidelity demodulation cannot be accomplished. This has heretofore been a problem.
Furthermore, in another example of the practice of the system of the above mentioned patent application, the AC lock range is fixed while the DC lock range is made large by maintaining the conversion gain of a voltage controlled oscillator in the PLL at a large DC value and at a small AC value (not especially large), thereby to attain a large lock range ratio.
By the practice of this example, however, if the lock range becomes excessively large, the PLL will be erroneous locked even by a signal of the sum signal band (less than 15 KHz). In this case, a signal within the sum signal band below 15 KHz will be demodulated as a disturbance wave. The demodulation output signal will become a signal containing a noise component in addition to the desired demodulated difference signal.
The following principal conditions are desirable for the demodulation of an angle modulated wave signal picked up from a multichannel disc and used in a system with a PLL.
1. The use of a large lock range ratio, for example, of the order of from 10:1 to 30:1.
2. The use of a maximum AC lock range of large value, for example, of the order of .+-. 10 KHz.
3. The use of a maximum DC lock range of suitable size, which, moreover, does not reach an unnecessary band (below 15 KHz). For this, the ideal lock range is of the order of from (30 - 14) KHz to (30 + 29) KHz. However, in actual practice, there is no problem if this range is from (30 + 19) KHz to (30 + 59) KHz.